Zoom viewfinder with non-rectangular window

ABSTRACT

A compact Galilean-type zoom finder suitable for compact cameras, and the like whose zoom ratio is about 2. The zoom finder is comprised of four units having negative, negative, positive and negative refractive powers in order from an object side and a movable framing window. The first and the fourth lens units are fixed and the second and the third lens units are moved to perform zooming. The moving framing window helps to delineate the field of view.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. Ser. No. 07/783,588, filed Oct. 28,1991, entitled ZOOM VIEWFINDER, in the name of Alan E. Lewis.

BACKGROUND OF THE INVENTION

This invention relates to zoom viewfinders.

Reverse Galilean-type viewfinders which include a projected frame orreticle easily delineate the field of view. When we eliminate thereticle or the frame, controlling the field of view becomes moredifficult. Merely changing magnification does not guarantee that as wezoom we will see the correct field of view (FOV), because the aperture(window) that limits the FOV may appear to change size resulting in avariation in the apparent FOV. Finally, if the optical system in aviewfinder has system distortion and pupil distortion, the field of viewwill not be properly delineated. An optical designer often attempts toeliminate the system distortion, but that comes at the expense of notminimizing other aberrations. In addition, pupil distortion may causethe clipping of the edges in the field of view if the aperture of thefield stop (i.e. framing window) is of the conventional rectangularshape.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to control thefield of view of a viewfinder.

The above and other objects are achieved by a viewfinder comprising aplurality of optical units and a field stop (i.e. framing window)located within said viewfinder, said field stop having fixed sizeaperture opening with curved sides for providing a rectangular field ofview. According to one aspect of the present invention, the apparentshape of the framing window aperture is controlled by having an actualaperture shape and size of the flaming window to match the system andpupil distortion of the optical system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a zoom viewfinder that forms a first embodimentof the invention;

FIG. 2 shows the margin control for the side of the format of the firstembodiment; and

FIG. 3 shows the shape of the aperture (i.e. framing window) of thefirst embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The lens system of the present invention has a general application tooptical systems employing viewfinders. An example of such application isa viewfinder suitable for use in compact cameras. In order to provide aconcise description of the preferred embodiment, certain details of theviewfinder and camera in which it may be used are not described hereinbut are selectable from the prior art.

The lens system forming the viewfinder of the first embodiment of thepresent invention will now be described with reference being made to theaccompanying FIG. 1. The lens system 100 is characterized by anunusually large half field angle (in a wide angle mode) of 32 degrees,2X zoom range and compactness in both length and front (object side)aperture. The viewfinder has high image quality and low distortion. Theouter elements of the zoom finder are fixed in position. This is helpfulfor keeping dust and dirt out of the mechanism. The lens system 100contains four optical units, these are in order from an object side, UI,UII, UIII and UIV. The lens 100 contains four airspaces A", B", C" andD" between the optical units UI, the framing window FW, the optical unitUII, the optical unit UIII and the optical unit UIV, respectively.Constant axial accommodation is achieved by use of two internaldifferentially moving (i.e. moving at different rates) optical units UIIand UIII. Thus, as UII and UIII move relative to UI during zooming, UIIalso moves relative to UIII.

Following from the object to pupil location the lens units are: Anegative power lens unit UI, which is stationary (i.e. it does not movefor zooming); a negative optical unit UII, movable along the opticalaxis for zooming; a positive optical unit UIII also movable along theoptical axis for zooming; and a stationary negative power optical unitUIV. The function of the front, negative optical unit UI is to expandthe beam diameter as it exists from a camera and enters the viewfinderand to correct distortion. Having the first optical unit UI and thesecond optical unit UII near each other allows coverage of the largestpossible angle in a wide angle mode. This is done by virtue of bringinga negative unit UII from the back of the optical system towards thefront of the optical system thus allowing more negative power towardsthe front in order to have maximum angular coverage. The positive poweroptical unit UIII is movable for zooming and is used for compensatingfor the drift in the position of the virtual image as seen by the eye ofthe viewer. It is located towards the rear of the optical system inorder to minimize the beam diameter. Finally, having a negative rearunit UIV, rather than it being positive, permits the positive unit UIIIto be made stronger, so that the optical unit UIII does not have totravel as far to achieve full zooming. In addition, the stationary unitUIV protects zooming groups from the foreign particular matter. Thecombination of units UIII and UIV act as an eyepiece in re-imaging thelight to the eye.

More specifically, the first lens unit UI comprises a negative lenselement L1 with a stronger curvature oriented towards a pupil position;the second lens unit UII comprises a negative meniscus lens element L2with a concave surface oriented toward to the first lens unit; the thirdlens unit UIII comprises a positive biconvex lens element L3; and thefourth lens unit UIV comprises a negative biconcave lens element L4. Allof the lens elements were made of plastic to make the production of theviewfinder relatively inexpensive. The numerical data for the opticalsystem 100 are as follows:

                  TABLE 1                                                         ______________________________________                                        Clear                                                                         Apertures                                                                     Top-       Side-                  Material                                    Surface                                                                              Bottom  Side   Radius  Thickness                                                                             N    V                                  ______________________________________                                        S1     8.64    12.59  469.668 1.500   1.492                                                                              574                                S2     6.90    10.08  ASPHERE A"                                              S3                    WINDOW  B"                                              S4     4.74    7.22   ASPHERE 1.700   1.535                                                                              405                                S5     5.37    8.22   -13.6721                                                                              C"                                              S6     6.67    10.38  12.4215 4.380   1.492                                                                              574                                S7     6.51    10.15  ASPHERE D"                                              S8     3.00    4.61   -121.781                                                                              1.200   1.535                                                                              405                                S9     2.94    4.25   22.0597                                                 FINDER LENGTH = 32.000                                                        ______________________________________                                        ASPHERIC EQUATION:                                                             ##STR1##                                                                     SURFACE S2 C = .14761      E = .12428E-05                                                k = -.87442     F = -.12197E-07                                               D = -.20684E-03 G = .84344E-10                                     SURFACE S4 C = -.16584     E = .45545E-05                                                k = -1.57580    F = .69169E-07                                                D = -.72436E-03 G = .15309E-10                                     SURFACE S7 C = -.07622     E = .11410E-05                                                k = -2.67459    F = -.14805E-07                                               D = .13448E-03  G = .57856E-10                                     ______________________________________                                        ZOOM THICKNESSES:                                                             MAG.     A"      B"            C"   D"                                        ______________________________________                                        .286     11.454  6.432         4.834                                                                              0.500                                     .389     7.147   5.541         4.435                                                                              6.097                                     .530     4.026   2.500         4.962                                                                              11.732                                    ______________________________________                                    

The space C' between the optical units II and III in the aboveembodiment is about 17% of the total finder length L. This appears toallow for superior performance for this type of lens system whilemaintaining the compactness of the overall system. The movement of UIIand UIII relative to each other and to the stationary units UI and UIVprovides a virtual image plane that is stationary and thus there is noneed for the eye to adjust its focus as the finder is zooming. If thespacing C" between units UII and UIII is kept constant so that theseinner units move together at the same rate (for simplified mechanicalconsiderations), the axial accommodation will vary with zoom position,but the image will still be viewable.

In the optical system 100, the field of view may be controlled by theaperture of the lens element L1 in the wide angle mode of operation. Theaperture of the lens element is defined by the periphery of theoptically transparent portion of the lens element or an opaque wall ofthe element mount, whichever is smaller. In the telephoto mode, anaperture on lens element L2 may be used to control the field of view.However, in the intermediate zoom positions, one of these two limitingapertures delineates the field of view, but the apparent field of viewvaries from its value at the two extreme zoom positions. If accuratecontrol of the apparent FOV is desired, a variable size aperture can beplaced near the front of the viewfinder as far from the eye as possibleto control the apparent field of view. However, the mechanism to changethe aperture size would require the front of the viewfinder to becomelarger in diameter which may work against a required goal ofcompactness. A better way to control the apparent field of view in areversed Galilean-type viewfinder was invented and is discussed below.

It was discovered that the placement of a movable field stop (i.e.framing window) FW having a fixed size aperture within the lens systemcontrols the margin in the intermediate zooming positions (see FIG. 2).Margin represents the percentage of the FOV of the final photographicprint that is seen through the finder. Margin control accommodates avariety of manufacturing and processing errors to assure that everythingseen through the viewfinder will be contained in the final photograph.In this embodiment, a moving framing window FW was placed between anegative power unit UI and a negative movable optical unit UII.

The position of this moving field stop (or framing window) is chosen tobe as far from the eye as possible to improve the sharpness of the edgesof the field as defined by the aperture of the framing window FW. Themoving framing window FW controls the field of view in all zoompositions to the desired 82% margin for the side of the format. Theaperture shape of the framing window is illustrated in FIG. 3 and has a"pin-cushion shape" of the following dimensions:

    W.sub.in =4.452; W.sub.out =4.778; L.sub.in =6.910; L.sub.out =7.200,

where W is the width and L is the length of the framing window aperture.Keeping the system distortion (of the lens system 100) throughout thezoom range of the same sign and similar in magnitude helps to keep thedesired apparent window aperture shape (defining the rectangular fieldof view) the same through the zoom range. The lens system 100 hasmaximum absolute value of system distortion of from 2.4% to 2.9%distortion at the edge of the field. Having residual distortion is notnecessary for the fixed size moving framing window concept to work.However, even when there is some system distortion present through thezoom range, matching the shape of the framing window aperture to thepupil distortion enables one to maintain a constant rectangular shape asseen by the eye (i.e. a constant rectangular object side FOV). Althoughthe lens format determines the field of view and thus the basic size ofthe field stop (i.e. framing window) aperture, it is pupil distortionand system distortion that effects the shape of the framing window'saperture.

In a Reverse-Galilean viewfinder, the negative lens out front willtypically introduce a positive (i.e. pin-cushion shape) pupil distortionand thus the aperture of the field stop (i.e. framing window's aperture)will typically be of a pin-cushion shape. This is especially true if thesystem distortion amount is -3% to +3%. Positive system distortion willmake the pin-cushion shape of the aperture of the framing window morepronounced, negative system distortion makes the pin-cushion shape ofthe aperture less pronounced. The function of the field mask (or framingwindow) is to simply delineate the field of view. The shape of the fieldstop (or framing window) aperture does not effect the system distortion.Since the overall optical system still has distortion, the viewing scene(buildings, etc.) may still appear distorted if the optical system has alarge amount of distortion (i.e. >|4%|). However, the shape of theaperture does effect the shape of the field. If the aperture isrectangular and the optical system suffers from the pupil distortion,the viewer may not see the entire field of view.

For example, in the preferred embodiment, the front negative lenselement introduces a positive pupil distortion. If the aperture of theframing window is rectangular shaped, it will cut out (i.e. vignette)the light coming through the corners, thus, darkening the edges of thefield. Thus, we compensated for the pupil distortion (as measured fromthe eye) caused by the strong negative front element by allowing theentire field of view to be unvignetted. This is done by making theaperture of the field stop (i.e. framing window) of a pin-cushion shape.0f course, in a different viewfinder system, a front element may bepositive and thus may introduce a strong negative (i.e. barrel-shapedpupil distortion), thus necessitating a different shape field stopaperture. We arrived at the proper shape of the field stop (or framingwindow) aperture by following a procedure knowing the format size of thesystem and thus knowing the field of view:

A. Three extreme rays were traced through the system backward from theeye. These rays corresponded to the top, side and the corner field ofview and were traced for the 3 zoom positions. First, the rayscorresponding to the top T and the side S (see FIG. 3) of the formatwere traced. They were verified by their exiting angles (which mustequal to the top and side half field of view) as the proper field rays.It was also verified that the two extreme top and side rays intersectedthe surface corresponding to the framing window at the same locationalong the optical axis. These intersections correspond to the framingwindow's aperture at mid-top and mid-side points. Next, athree-dimensional ray trace of the corner ray was done. The ray wastraced from the corner of the eye pupil at an angle determined by theformat of the FOV of the camera. When the corner ray emerged out of theoptical system, the horizontal and vertical components of the angle ofthe emerging ray should be the same as for the top and the side rays. Ifthese components do not match the exiting angles for the top and theside rays, the slightly different ray has to be traced again. Thisiterative process has to be repeated until the corner ray componentssubstantially match those of the top and side ray.

B. The coordinates of the corner ray at the framing window locationshould then be determined. The coordinates define the location of thecorners for the field mask.

C. Once the location of the framing window's mid-top, mid-side andcorner coordinates are known, we can connect them (by assuming that acircle defines the curve that connects them together).

D. In a non-zooming viewfinder, or in a viewfinder system that haslittle distortion (as shown in a preferred embodiment), this proceduredefines the shape of the aperture of the framing window. In a zoomingviewfinder that has a wide range of system distortion, one may have totake the average coordinates (for the top, corner and the side rays)from two extreme zoom positions and use them to define the shape of theaperture.

FIG. 2 shows the margin for the side of the format in lens system 100assuming only the moving field stop aperture (or framing windowaperture) determines the FOV. The top and bottom margin is alsoprecisely controlled. Thus, for lens system 100, the entire apparentfield of view is precisely controlled by moving a framing window havinga fixed size aperture.

The invention has been described in detail with particular reference toa preferred embodiment thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

I Claim:
 1. A viewfinder comprising a plurality of optical units and aframing window located within said viewfinder, said framing windowhaving a fixed size non-rectangular aperture opening with curved sidesto provide a rectangular field of view.
 2. A viewfinder according toclaim 1 wherein said viewfinder is a zoom viewfinder and wherein saidframing window moves during zooming.
 3. A viewfinder comprising aplurality of optical units and a framing window located within saidviewfinder between two of said optical units, said framing window havinga fixed size non-rectangular aperture opening with curved sides forproviding a rectangular field of view.
 4. A viewfinder according toclaim 3 wherein said viewfinder is a zoom viewfinder and wherein saidframing window moves during zooming.
 5. A viewfinder according to claim4 wherein said viewfinder has curved sides to match distortion forproviding a rectangular field of view.
 6. A viewfinder according toclaim 1 wherein said aperture opening has pin-cushion shape.
 7. Aviewfinder according to claim 2 wherein said aperture opening haspin-cushion shape.
 8. A viewfinder according to claim 3 wherein saidaperture opening has pin-cushion shape.
 9. A viewfinder according toclaim 4 wherein said aperture opening has pin-cushion shape.
 10. Aviewfinder according to claim 5 wherein said aperture opening haspin-cushion shape.